This invention relates to a recording/playback apparatus which performs at least recording of a prescribed signal on a rotating magnetic, optical or other recording medium in synchronization with its rotational reference phase, or playback of a signal from the rotating recording medium in synchronization with its rotational reference phase. More particularly, the invention relates to a system in which control of such a recording/playback apparatus is shared by a plurality of controllers (inclusive of CPU's). The invention relates also to a method of communication among the plurality of controllers in the recording/playback apparatus.
Typical examples of a recording/playback apparatus of the kind mentioned above include a still video camera which forms a still image of a subject by means of a solid-state electronic image pick-up device, frequency-modulates the still video output of the device and magnetically records the modulated signal on a rotating video floppy, a playback apparatus which plays back the still video signal from a video floppy, and an apparatus which possess both of these recording and playback functions.
In such a still video signal recording/playback apparatus, one field of a still video signal is recorded and/or played back by one revolution of the video floppy. A reference angle position for recording/playback of the still video signal relative to the video floppy is predetermined and is detected by a phase detector. A reference signal (a vertical synchronizing signal or the like) is formed for the purpose of recording/playback synchronized to phase pulses outputted by the phase detector. The principal operation of the recording/playback apparatus is performed with this reference signal serving as the overall reference.
A disk motor for rotatively driving the video floppy is controlled by a servo-control circuit so as to rotate precisely at a fixed rotational speed (rpm). Any fluctuation in the rotational speed will result in recording/playback of the still video signal being performed incorrectly. This makes it necessary to check, at all times, whether the rotational speed of the disk motor is being held at the predetermined fixed value. Such an operation is referred to as servo-lock decision processing.
Processing (synchronized phase relationship decision processing) for synchronizing the phase pulses representing the rotational phase of the video floppy and the reference signal of the apparatus so that a predetermined phase relationship will be maintained between them, or for performing a check to see whether the predetermined phase relationship is being maintained, as well as the abovementioned servo-lock decision processing, includes processing for measuring very small periods of time. For this reason, very high precision is required.
Meanwhile, consider an arrangement in which control of a recording/playback apparatus is shared by a plurality of controllers (inclusive of CPU's). In such a case, it will be necessary for the controllers to communicate with one another. In such an arrangement, a communication request is generated separately by each controller, and it is required that such a request be responded to promptly. In general, therefore, control of communication among the plurality of controllers is given a high order of priority.
However, if an interrupt for communication purposes arrives and a controller proceeds to the interrupt processing routine during the course of synchronized phase relationship decision processing or servo-lock decision processing, there is the danger that the high precision of this decision processing will not be able to be maintained. Accordingly, it is preferred that the controllers be forbidden from executing communication processing during the time that the abovementioned decision processing is being carried out.
The abovementioned recording/playback apparatus comprises a multiple-CPU system constituted by a main CPU (master CPU) and subordinate CPU's (slave CPU's) connected thereto. With such a multiple-CPU arrangement, measures must be taken to deal with an instantaneous-shutdown phenomenon.
The term "instantaneous shutdown" refers to an instantaneous drop in power supply voltage and has a variety of causes. For example, in a case where a multiple-CPU system is powered by a battery, the system will come to an instantaneous shutdown if contact with the battery terminals is temporarily lost due to vibration or the like. Also, there are situations in which the supply voltage drops when an excessively large current flows through the electrical components (e.g., motor and solenoids) controlled by the multiple-CPU system.
To cope with this instantaneous-shutdown phenomenon, a circuit for detecting the phenomenon is provided and the CPU's assume a standby state when the phenomenon is detected. This prevents CPU malfunction and runaway ascribable to instantaneous shutdown.
Since the instantaneous-shutdown phenomenon is a drop in power supply voltage, it is preferred that the multiple-CPU system be arranged so as to resume operation as soon as the power supply voltage is restored. Moreover, it would be desirable for this resumption of operation to take place automatically without requiring, to the maximum degree possible, an operator input from the outside.